1. Field of the Invention
The present invention relates to a bend sensor for detecting the degree of bend of an elastic plate.
2. Related Background Art
There has been conventionally a bend sensor for detecting the degree of bend of an elastic plate by detecting dimensional variations generated at an obverse and a reverse of the elastic plate when the elastic plate obtained by mutually sticking, at bottom surfaces thereof, two base members made of a plate-like elastic member having a strain gage or a thick film resistor formed thereon, respectively, is bent.
A general discussion will be first conducted on the principle of a bend sensor.
FIG. 8 is a side view schematically showing an elastic plate constituting a bend sensor. FIG. 9 is a side view showing a state in which the elastic plate is bent.
As shown in FIG. 8, it is assumed that each of an obverse side A1B1 and a reverse side C1D1 of an elastic plate 100 has a length L1 in an initial state.
An external force is exerted on the elastic plate 100, which is then curved at a radius r of curvature on a neutral line of a sensor, as shown in FIG. 9. At this time, it is assumed that an arc A2B2 of the curved side A1B1 has a length L2 (>L1) while an arc C2D2 of the curved side C1D1 has a length L3 (<L1).
Here, a strain εAB at the obverse of the elastic plate caused by the bend is expressed by the following equation:εAB=ΔL2/L1=(L2−L1)/L1   (1)
The right side of the equation is expressed by using the radius r of curvature on the neutral line, as follows:
                                                                        ɛ                AB                            =                                                                    (                                                                  L                        ⁢                                                                                                  ⁢                        2                                            -                                              L                        ⁢                                                                                                  ⁢                        1                                                              )                                    /                  L                                ⁢                                                                  ⁢                1                                                                                        =                                                                    [                                                                  2                        ⁢                        π                        ⁢                                                  {                                                                                    (                                                              t                                /                                2                                                            )                                                        +                            r                                                    }                                                                    -                                              2                        ⁢                        π                        ⁢                                                                                                  ⁢                        r                                                              ]                                    /                  2                                ⁢                π                ⁢                                                                  ⁢                r                                                                                        =                              π                ⁢                                                                  ⁢                                  t                  /                  2                                ⁢                π                ⁢                                                                  ⁢                r                                                                        (        2        )            
Similarly, a strain εCD at the reverse of the elastic plate caused by the bend is expressed by the following equation:εCD=ΔL3/L1=(L3−L1)/L1   (3)
The right side of the equation is expressed by using the radius r of curvature on the neutral line, as follows:
                                                                        ɛ                CD                            =                                                                    (                                                                  L                        ⁢                                                                                                  ⁢                        3                                            -                                              L                        ⁢                                                                                                  ⁢                        1                                                              )                                    /                  L                                ⁢                                                                  ⁢                1                                                                                        =                                                                    [                                                                  2                        ⁢                        π                        ⁢                                                  {                                                                                    -                                                              (                                                                  t                                  /                                  2                                                                )                                                                                      +                            r                                                    }                                                                    -                                              2                        ⁢                        π                        ⁢                                                                                                  ⁢                        r                                                              ]                                    /                  2                                ⁢                π                ⁢                                                                  ⁢                r                                                                                        =                                                -                  π                                ⁢                                                                  ⁢                                  t                  /                  2                                ⁢                π                ⁢                                                                  ⁢                r                                                                                        =                              -                                  ɛ                  AB                                                                                        (        4        )            
As a consequence, in the case where a 4-active bridge circuit having two strain gages disposed at each of the obverse and reverse of the elastic plate, that is, four strain gages in total is configured, an output of one active bridge circuit corresponding to one strain gage is expressed by the following equation:e1=(E/4)K εAB where E represents an applied voltage of the entire 4-active bridge circuit, and further, K indicates a gage coefficient thereof. In consideration of a polarity of the voltage, the output of the 4-active bridge circuit is expressed by the following equation:e=4×e1=E K εAB 
As is obvious from the equations (2) and (4), as the radius r of bend curvature, or the radius r of curvature of the bend sensor becomes smaller, a magnitude (i.e., an absolute value) of each of the strains εAB and εCD becomes greater. And as a thickness t of the elastic plate becomes greater, the magnitude of each of the strains εAB and εCD becomes greater.
FIG. 10 is a graph illustrating the relationship between the radius r of curvature of the bend sensor and an output of the bend sensor.
As also illustrated in the graph of FIG. 10, as the radius r of bend curvature, or the radius r of curvature of the bend sensor becomes smaller, the magnitude (absolute value) of each of the strains εAB and εCD becomes greater.
One example of the conventional bend sensor in accordance with the above-described principle has been proposed and known by a bend sensor constituting a Wheatstone bridge circuit, in which elastic plates obtained by mutually sticking bottom surfaces of two base members made of a plate-like elastic member having a resistor variable in resistance in response to bend, that is, a strain, formed thereon are connected at the resistors thereof to each other. See, for example, Japanese Patent Laid-Open Publication No. 6-123604A.
However, in the conventional bend sensor obtained by mutually sticking, at bottom surfaces thereof, the two base members made of the plate-like elastic member having the strain gage or the thick film resistor formed thereon, respectively, a through hole need be bored in the elastic plate having the two base members stuck to each other, and then, strain sensitive elements need be connected to each other in order to constitute a bridge circuit by connecting the strain sensitive elements disposed at an obverse and a reverse to each other, thereby raising a problem of the necessity of a fabrication process therefor.
FIG. 11 is a side view showing a part of the conventional bend sensor, in which two base members, each having a strain sensitive element formed at an obverse thereof, are stuck to each other.
As shown in FIG. 11, the conventional bend sensor is constituted by mutually sticking a base member 41 having a strain sensitive element 51 formed thereon and another base member 42 having another strain sensitive element 52 formed thereon.
Therefore, in order to constitute a bridge circuit by connecting the strain sensitive elements 51 and 52 to each other, through holes need be bored in the base members 41 and 42 stuck to each other, and then, the strain sensitive elements 51 and 52 need be connected to each other. Accordingly, a fabrication process is required therefor.
Moreover, in sticking the base members 41 and 42 to each other, the obverse and reverse strain sensitive elements 51 and 52 need be accurately aligned each other, thereby requiring a time for a sticking process and degrading a yield.
Additionally, the conventional bend sensor has the structure in which the two base members 41 and 42 are stuck to each other, and therefore, the entire thickness of the elastic plate except the strain sensitive elements 51 and 52 becomes as large as t10+t20, wherein the thicknesses of the base members 41 and 42 are designated by t10 and t20, respectively. For example, although the base members 41 and 42, each having a thickness of 25 μm, can be formed, the entire thickness of the elastic plate except the strain sensitive elements 51 and 52 becomes twice, that is, as large as 50 μm by sticking the base members 41 and 42 to each other.
When the elastic plate receives bend at a small radius r of bend curvature, that is, when a large external force is exerted on the bend sensor in the case that the elastic plate constituting the bend sensor is thick, a strain at the strain sensitive elements 51 and 52 becomes excessive, thereby exerting a harmful effect of a shortage of a fatigue lifetime. For example, when bend at a radius r of bend curvature of 20 mm is applied to the elastic plate having a thickness of 50 μm, a strain at the strain sensitive elements 51 and 52 becomes about 2500 με, so that only several tens of thousands of bending times can be expected in regard to a fatigue lifetime. According to usage, the fatigue lifetime of the bend sensor is desired to be a million of bending times.